Optoelectronics: Photonic Materials and Devices
DT086/DT085
Fibre Bragg Grating (FBG) Sensing
Lab #1
Objective
To study operation of an optical fibre Bragg grating (FBG) sensor, to measure the
dependence of the Bragg wavelength shift versus applied static strain and to
compare the strain measured by the sensor with theoretically estimated strain.
Equipment
Optical Bench, Broadband optical source (SLD) and driver, Optical isolator, Optical
coupler, FBG sensor, Horizontal translation stage with a micro-screw, Stationary fibre
holder, Optical Spectrum Analyzer (OSA).
Theory
The basic operation of FBG strain sensor is based on the measurement of the peak
wavelength shift induced by the applied strain. The light reflected by periodic
variations of refractive index of the Bragg grating having a central wavelength B is
given by:
 B  2neff  ,
(1)
where neff is the effective refractive index of the core and  is the periodicity of the
refractive index modulation. When the strain is applied there will be a shift from this
peak wavelength, which can be calculated using the following formula:
 B   B (1   )
(2)
where  is the strain applied to the fibre,  is the photo elastic coefficient of the
neff 2
12   ( 11  12) , 11 and 12 are the
fibre given by the formula  
2
components of fibre optic strain sensor and  is the Poisson’s ratio. For silica core
fibre the value of (1 - ) is usually 0.78.
Procedure
To measure the strain applied to the fibre containing an FBG sensor you will use the
experimental setup shown in Figure 1. It consists of a super luminescent diode used
as a broadband source to interrogate the Bragg grating. An optical isolator protects
the super-luminiscent diode from the back-reflected light. 2x2 fibre optic coupler
directs the light from the source to the fibre with an FBG sensor and the light
reflected back from the sensor to the OSA for analysis. A piece of fibre containing an
FBG grating is fixed between the stationary fibre holder (at one end) and a translation
stage with a micro-screw (at another end) so the motion of the translation stage
causes stretching and elongation of the fibre.
FBG sensor
SLD
Isolator
2x2 coupler
Fibre
Stationary
holder
Horizontal
Translation
stage
OSA
Figure 1. Schematic diagram of the experimental setup.
1. Measure the Bragg wavelength of the fibre sensor B in an “unstrained” state.
2. Using the translational stage micro-screw start applying strain to the fibre in
small intervals, recording both shift of the Bragg wavelength and the value of
fibre elongation caused by the horizontal translation. NOTE: Application of
an excessive strain can cause breakage of the FBG! Consult laboratory
assistant for the translation stage motion limit.
3. Using Equation (2), find the value of strain  applied to the fibre containing
the FBG sensor.
4. At each position of the translation stage find an estimated value of the strain
as
L
 est 
L
where L is the fibre elongation (position of the micro-screw) and L is the
length of the fibre piece between the stationary holder and horizontal
translation stage (we can apply this formula because strain is defined as the
fractional change in length)
5. Fill in the following table:
Displacement
Bragg
(position of the wavelength shift,
micro-screw),
B, pm
L, m
Measured
strain,
, 
Estimated Error,
strain
I-estI/
est, 
6. Plot the measured and estimated strains versus fibre elongation.
7. Which technique is more accurate? Explain your answer.
8. From the experiment find the value of sensitivity (Bragg wavelength shift per
 of strain) of the FBG sensor to strain in the studied range. Explain how you
achieved the result and compare your result with a typical literature reference
value.